Field of the Invention
This invention relates to the production of iminodiacetonitrile, and more specifically to an integrated process wherein a crude, unpurified reactor gas stream from a hydrogen cyanide reactor and optionally, a crude, unpurified reactor gas stream from a formaldehyde process reactor are fed directly to a reactive absorber together with additional ammonia and acidified water to produce iminodiacetonitrile in high yields. This process provides improved economics for producing iminodiacetonitrile by eliminating costly intermediate recovery and purification processes associated with conventional hydrogen cyanide and formaldehyde production processes.
It is known in the prior art that iminodiacetonitrile can be prepared by reacting formaldehyde, hydrogen cyanide and ammonia in aqueous solutions. While the order of the reactant addition may vary in practice, the overall reaction can be written as follows: ##STR1##
On an industrial scale, the conventional process for the production of iminodiacetonitrile is reliant upon commercially available, purified hydrogen cyanide and formaldehyde reactants.
Commercially available hydrogen cyanide is produced primarily by the ammoxidation of methane (Andrussow Process or the Degussa process, also called the BMA process), the reaction of ammonia and propane (Fluohmic process), the ammoxidation of methanol, the decomposition of formamide, and the recovery of hydrogen cyanide as the by-product in the preparation of acrylonitrile by the ammoxidation of propylene (SOHIO process). These and other similar processes are well documented in the art. Since all of these processes either use ammonia as the source of nitrogen or have ammonia present as a by-product, the hydrogen cyanide reactor product gas streams contain some unreacted ammonia. This unreacted ammonia must be removed before recovering hydrogen cyanide to avoid dangerous exothermic polymerization of the liquid hydrogen cyanide. Therefore each of these processes must employ a general three-stage process wherein:
(1) a crude, dilute gaseous hydrogen cyanide product stream is formed by one of the above listed methods, product PA0 (2) excess unreacted ammonia in the reactor product gas stream is removed, and recovered by selective absorption and stripping, and PA0 (3) purified hydrogen cyanide is obtained by water scrubbing followed by stripping out the water solvent. (See FIG. 1)
Each of these three stages in the manufacturing process has substantial capital requirements, which in turn requires a substantially large scale operation for any of these processes to be economically feasible.
The predominant processes for the production of commercial grade formaldehyde are by the dehydrogenation of methanol over silver catalyst (BASF process) or by the oxidation of methanol over a metal oxide catalyst such as ferric molybdate.
The reaction using the silver catalyst is endothermic, and may be written as follows: EQU CH.sub.3 OH.fwdarw.CH.sub.2 O+H.sub.2
The reaction using ferric molybdate catalyst is exothermic, and may be written as follows: EQU CH.sub.3 OH+1/2O.sub.2 .fwdarw.CH.sub.2 O+H.sub.2 O
Both of the above commercial processes employ a general two-stage process wherein:
(1) A crude, dilute formaldehyde product stream is produced, and
(2) formaldehyde is recovered by aqueous absorption columns.
Substantial capital equipment costs can be attributed to the large absorption columns required to obtain commercial grade formaldehyde, which therefore requires a large scale production plant to afford economically feasibility.
Under the process of this invention, much of these recovery and purification equipment costs can be eliminated. That is, in the production of hydrogen cyanide, the crude reactor product stream is a mixture containing, in addition to hydrogen cyanide, a significant amount of ammonia and excess water. Similarly, in the production of formaldehyde, the crude reactor product stream contains, in addition to formaldehyde, a significant amount of excess water. Under the process of this invention, it has been discovered that it is not necessary, and in fact, is actually redundant to first remove this unreacted ammonia and/or excess water from the crude reactor gas product streams, only to reintroduce it in the down-stream reactor when forming iminodiacetonitrile. This is because it has now been found that it is possible to combine these crude reactor product streams directly to achieve high yields of iminodiacetonitrile product. The high yields obtained by this invention were unexpected and not obvious since similar attempts to produce other acetonitrile products such as nitrilotriacetonitrile or ethylenediamintetraacetonitrile using this same process were unsuccessful. For example, crude reactor streams containing formaldehyde and hydrogen cyanide were produced and fed directly into a reactive absorber together with an additional ammonia source. When scrubbed with a solution of pH 0-1, a range that favored the production of nitrilotriacetonitrile, a yield of less than 40% was obtained. A similar process, utilized the addition of ethylenediamine in the place of ammonia, and a scrubbing solution of pH=0.8 which favored the production of ethylendiaminetetracetonitrile, produced yields of approximately 25%. In comparison, this process as described in the detailed description, when applied to the production of iminodiacetonitrile, produced yields in excess of 80%.